s parameter input to impedance vs time plot

Hi, I have been trying to create impedance vs time plot for s-parameter input without much luck. The s-parameter input is measurement .s4p file, I would also like to convert simulation generated .sNp files as well.

I donot have RF toolbox and have just floating matlab license with me. I have written a code using ifft but cannot get it to work. Let me know and I can send you my code. Can you please help me in generating the plot?

Thanks, R