  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 response time decreases for thinner metal films. The
macroscopic response time depends on the mean and the variance of the
distribution of microscopic momentum relaxation times of the conducting
electrons. Comparing the recorded response times with DC-conductivities
demonstrates increasing variance of the microscopic relaxation times with
increasing film thickness. At least two electron species contribute to
conduction in bulk with substantially differing relaxation times. The different
electron species are affected differently by the confinement because they have
different mean free paths.
