The thermal conductivity is given by κ κ = c V τ τ ⟨ ⟨ v 2 ⟩ ⟩ / 3 for free particles, which is proportional to the heat capacity and the mean free path which depend on the model ( ⟨ ⟨ v 2 ⟩ ⟩ 1 / 2 is the mean (square) speed of the electrons or the Fermi speed in the case of the free electron model). This implies that the ratio between thermal and electric conductivity is given by the Wiedemann–Franz law,