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pointed me to

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html
(which was extremely helpful to me in understanding what was going on here).

Working out tau as d, the mean free path, divided by the fermi velocity vf,

gives the same answer the first website gave, tau = 2.5e-14 sec, which

corresponds to f=1/tau = 40 Thz or 7500nm, about a tenth the frequency of

visible light.

But you may be right - I seem to recall that the rate of change of momentum

with velocity is not equal to the bare electron mass in a crystal latice

(but this was from some old semiconductor courses, I don't know if metals

have a regular enough crystal structure for this analysis to be applied

correctly).