  We measure the electron conductivity of the surface states and the subsurface
space charge layer originating from the Si(111)-4x1-In reconstruction as a
function of temperature. The conductivity of the surface states drops sharply
around 130 K with decreasing temperature, revealing a metal-insulator phase
transition of the surface reconstruction. In contrast, the influence of the
phase transition on the conductivity of the space charge layer is limited to
temperatures above 60 K. This means that the surface Fermi level remains
strongly pinned despite the phase transition, indicating the presence of free
carriers in the surface states down to rather low temperatures.
