Relaxation time = displacement lifetime of fermi sphere?

[neu]

Homework Statement

I just want to clear this up, I am a little confused:

when an electric field is applied there is a force on the electron K-states thus displacing the fermi surface/sphere

Is the relaxation time ($$\tau$$)= lifetime of fermi sphere displacement

or is lifetime of displacement= reciprocal relaxation time

both are labebel as $$\tau$$ in my notes and related as below but how can they be the same?

Homework Equations

$$\sigma = \frac{n e^2 \tau}{m}$$ electrical conductivity

$$\rho = \frac{1}{\sigma}=\frac{m}{n e^2 \tau}$$ electrical resistivity

 

[solas99]

Given the following data on copper, how do i calculate the resistivity?

Relaxation time: 2.50e10-14s
Density: 8940Kgm-3
molar mass: 63.5g

is there an equation for it.