parameter estimation using MATLAB

Having problems. I have test data and i want to estimate the parameters which will give me similar curve. I dont know what to do. Please help. Attached is my code. % ---- Main Code ------ % % First guess for parameters p0 = [10000, 1e-12, 50000, 20000];

% Load data
 data = load('dev5_a.dat', 'ascii');

% First column is frequency in Hz, convert to rad/sec
freq = 2*pi*data(:, 1);

% Second column is magnitude impedance in Ohms
data(:,2) = (data(:,2));
% Third column is impedance angle in deg, convert to rads
data(:, 3) = data(:, 3);

z = [data(:,2) data(:,3)];

p0 = fminsearch(@(p0) costfun2(p0,z,freq),...
     p0) ;
new = origmodel(p0,freq) ;

       % plot original
  close all
  figure
  %scatter(data(:,1),data(:,2))
  plot(data(:,1),log(data(:,2)),'b')
  % Plot estimate
  hold on
  %scatter(data(:,1),new(:,1))
  plot(data(:,1),log(new(:,1)),'r')

% -- Calling function ----
function [out] = costfun2(p,z, f)

R1 = (p(1)); %*1e3;
C1 = (p(2)); %*1e-12;
R2 = (p(3)); %*1e3;
C2 = (p(4)); %*1e-12;

z1 = R1 - i./(f*C1);
z2_inv = (1/R2+1./z1);
z2 = (z2_inv).^-1;
zt = z2 - i./(f*C2);

znew(:, 1) = (abs(zt));
znew(:, 2) = angle(zt);
er = z(:) - znew(:);
out = er'*er;
txt = sprintf('The error is %f',out);
disp(txt)

% --- orig model ----
function [z] = origmodel(p, f)

R1 = (p(1)); %*1e3;
C1 = (p(2)); %*1e-12;
R2 = (p(3)); %*1e3;
C2 = (p(4)); %*1e-12;

z1 = R1 - i./(f*C1);
z2_inv = (1/R2+1./z1);
z2 = (z2_inv).^-1;
zt = z2 - i./(f*C2);

z(:, 1) = (abs(zt));

z(:, 2) = angle(zt);

return;

%%%%%------- Test DATA -----------
% Freq. [Hz]  |Z| [Ohm]  Angle [deg]
2.00000E+1  2.17111E+4  -4.97566E+1
2.40000E+1  1.95677E+4  -5.09048E+1
2.88000E+1  1.75852E+4  -5.18520E+1
3.45600E+1  1.57614E+4  -5.25666E+1
4.14720E+1  1.41032E+4  -5.31513E+1
4.97664E+1  1.25956E+4  -5.35962E+1
5.97197E+1  1.12928E+4  -5.40219E+1
7.16636E+1  1.00903E+4  -5.41847E+1
8.59963E+1  9.04188E+3  -5.43254E+1
1.03196E+2  8.09931E+3  -5.43871E+1
1.23835E+2  7.25300E+3  -5.43497E+1
1.48602E+2  6.49768E+3  -5.41649E+1
1.78322E+2  5.83313E+3  -5.38856E+1
2.13986E+2  5.23557E+3  -5.35058E+1
2.56784E+2  4.70178E+3  -5.30118E+1
3.08140E+2  4.24002E+3  -5.24886E+1
3.69769E+2  3.82403E+3  -5.19025E+1
4.43722E+2  3.45443E+3  -5.12147E+1
5.32467E+2  3.12396E+3  -5.04663E+1
6.38960E+2  2.82972E+3  -4.96907E+1
7.66752E+2  2.56319E+3  -4.88257E+1
9.20102E+2  2.32655E+3  -4.78864E+1
1.10294E+3  2.11354E+3  -4.68502E+1
1.32979E+3  1.92099E+3  -4.57067E+1
1.59574E+3  1.75442E+3  -4.44931E+1
1.89394E+3  1.61488E+3  -4.32674E+1
2.27273E+3  1.48251E+3  -4.18808E+1
2.72727E+3  1.36475E+3  -4.03953E+1
3.28947E+3  1.25807E+3  -3.87924E+1
3.94474E+3  1.16634E+3  -3.71557E+1
4.68750E+3  1.08959E+3  -3.55618E+1
5.68182E+3  1.01377E+3  -3.37151E+1
6.81818E+3  9.51390E+2  -3.19480E+1
8.33333E+3  8.91081E+2  -2.99645E+1
1.00000E+4  8.43392E+2  -2.81755E+1
1.19084E+4  8.03597E+2  -2.65023E+1
1.41176E+4  7.69179E+2  -2.48993E+1
1.71429E+4  7.34613E+2  -2.31238E+1
2.06897E+4  7.05560E+2  -2.14962E+1
2.40000E+4  6.85431E+2  -2.02984E+1
2.94118E+4  6.60834E+2  -1.87544E+1
3.52941E+4  6.41529E+2  -1.74947E+1
4.28571E+4  6.23334E+2  -1.63007E+1
5.00000E+4  6.10356E+2  -1.54652E+1
6.00000E+4  5.96319E+2  -1.46004E+1
7.24286E+4  5.84014E+2  -1.38975E+1
8.57143E+4  5.72033E+2  -1.32940E+1
1.00000E+5  5.62451E+2  -1.28862E+1
1.25000E+5  5.49142E+2  -1.24387E+1
1.50000E+5  5.38642E+2  -1.22093E+1
1.66667E+5  5.32645E+2  -1.21344E+1
2.00000E+5  5.22345E+2  -1.20683E+1
2.50000E+5  5.09576E+2  -1.21084E+1
3.20000E+5  4.95257E+2  -1.22910E+1
4.00000E+5  4.81978E+2  -1.25578E+1
4.80000E+5  4.70751E+2  -1.28357E+1
5.00000E+5  4.68159E+2  -1.29117E+1
6.40000E+5  4.52411E+2  -1.33648E+1
8.00000E+5  4.37780E+2  -1.38281E+1
9.60000E+5  4.25443E+2  -1.42423E+1
1.00000E+6  4.22626E+2  -1.43434E+1