If we simply use the interatomic distance between copper atoms as the
mean free path L for electrons, and vF for vrms in ne 2L/mevrms, we find a
value for that is about 200 times smaller than the measured value of the conductivity.
This discrepancy implies that we are using the wrong value for L and
that the scattering sites for electrons are not adjacent ion cores but more
widely separated scattering centers.