A plot conductivity vs. 1/temperature looks pretty much like the plot carrier concentration vs 1/temperature in problem 17.2.
I think that's because in the free electron model, conductivity is proportional to n.
However, at large 1/T the conductivity increases again.
Why? Does the scattering time depende on temperature?
Conductivity is proportional to n*tau. Tau is again proportional to 1/n. So why is the temperature dependence of n important for the temperature dependence of the conductivity?