  Terahertz time-domain conductivity measurements in 2 to 100 nm thick iron
films resolve the femtosecond time delay between applied electric fields and
resulting currents. This current response time decreases from 29 fs for
thickest films to 7 fs for the thinnest films. The macroscopic response time is
not strictly proportional to the conductivity. This excludes the existence of a
single relaxation time universal for all conduction electrons. We must assume a
distribution of microscopic momentum relaxation times. The macroscopic response
time depends on average and variation of this distribution; the observed
deviation between response time and conductivity scaling corresponds to the
scaling of the variation. The variation of microscopic relaxation times depends
on film thickness because electrons with different relaxation times are
affected differently by the confinement since they have different mean free
paths.
