The major factor determining transport properties of solids is the number of electron states in a vicinity of Fermi level. In equilibrium, no macroscopic flow of electrons exists and, therefore, in order to create such flow, electrons must be excited over their equilibrium distribution. However, electrons with energies well below the Fermi level (compared to the characteristic energy scale kBT, where kB is the Boltzmann constant and T is the temperature), cannot acquire small excitation energy. Indeed, in this case, they would have energy corresponding to already occupied states, which is prohibited by the Pauli principle. In turn, well above the Fermi level, where excitation of electrons is not constrained by the Pauli principle, the electron states are not populated, thus making their contribution to the response negligible.