The value L 0 = 2.44×10 V K results from the fact that at low temperatures ( T → → 0 K) the heat and charge currents are carried by the same quasi-particles: electrons or holes. At finite temperatures two mechanisms produce a deviation of the ratio L = κ κ / ( σ σ T ) from the theoretical Lorenz value L 0 : (i) other thermal carriers such as phonons or magnons, (ii) Inelastic scattering. As the temperature tends to 0 K, inelastic scattering becomes weak and promotes large q scattering values (trajectory a in the figure). For each electron transported, a thermal excitation is also carried and the Lorenz number is reached L = L 0. Note that in a perfect metal, inelastic scattering would be completely absent in the limit T → → 0 K and the thermal conductivity would vanish κ κ → → 0 ; L → → 0 . At finite temperature small q scattering values are possible (trajectory b in the figure) and electrons can be transported without the transport of a thermal excitation L (T) < L 0. At higher temperatures, the contribution of phonons to thermal transport in a system becomes important. This can lead to L (T) > L 0. Above the Debye temperature the phonon contribution to thermal transport is constant and the ratio L (T) is again found constant.