A couple of things:
Make a habit of ending lines with a semicolon (;) to suppress the output. It doesn't make a big difference now, but later down the road with big data, printing the output will slow down processing.
Your R value is being read as 50 instead of 50000. If there are too many zeros for pleasant reading, you could type R = 50e+3.
Try semilogy instead of plot to show this data.
R = 50000;
C = 2200e-12;
imax = 1/R;
tau = R*C;
t = [0:.0001:1];
current = imax*exp(-t ./ tau);
axis ([.00000001 .01 0 .01]);
semilogy(t, current)
