This is the explanation for the Wiedemann-Franz law
I just had a look at this site. According to this site clasical and qm theory suggest that KE of (roughly) 3kT is absorbed by each conduction electron. Since the total absorbtion per atom is also about 3kT, can I then draw the conclusion that no energy is transferred to the +ve ion?
What happens when we have more than one free electron per atom?