  Electronic transport properties of the Si(111)-$(\sqrt{3}\times\sqrt{3})$-Sn
surface formed on low doped Si substrates are studied using two-probe
conductivity measurements and tunnelling spectroscopy. We demonstrate that the
ground state corresponds to Mott-Hubbard insulator with a band gap $2\Delta =
70$meV, which vanishes quickly upon temperature increase. The temperature
dependence of the surface conductivity above $T > 50$K corresponds to the
Efros-Shklovskii hopping conduction law. The energy gap at the Fermi level
observed in tunnelling spectroscopy measurements at higher temperatures could
be described in terms of dynamic Coulomb blockade approximation. The obtained
localization length of electron is $\xi = 7$\AA.
