Derive the WiedemannFranz law (5.8.21) using the definitions of the electrical conductivity, the thermal conductivity, and the

Question:

Derive the Wiedemann–Franz law (5.8.21) using the definitions of the electrical conductivity, the thermal conductivity, and the diffusion constant given in the previous sections, along with the Sommerfeld relation (4.9.31) for the heat capacity of the electron gas, and the diffusion constant for a degenerate Fermi gas.