Direct experimental confirmation of the gap has been done via tunneling experiments, which probed the single-particle DOS in two and three dimensions. The experiments clearly showed a linear gap in two dimensions, and a parabolic gap in three dimensions. Another experimental consequence of the Coulomb gap is found in the conductivity of samples in the localized regime. The existence of a gap in the spectrum of excitations would result in a lowered conductivity than that predicted by Mott variable-range hopping. If one uses the analytical expression of the single-particle DOS in the Mott derivation, a universal e − − 1 / T 1 / 2 dependence is obtained, for any dimension. The observation of this is expected to occur below a certain temperature, such that the optimal energy of hopping would be smaller than the width of the Coulomb gap. The transition from Mott to so-called Efros–Shklovskii variable-range hopping has been observed experimentally for various systems. Nevertheless, no rigorous derivation of the Efros–Shklovskii conductivity formula has been put forth, and in some experiments e − − 1 / T α α behavior is observed, with a value of α α that fits neither the Mott nor the Efros–Shklovskii theories.