Non-ohmic conductivity of barely localized electrons in three dimensions

We observe a rapid temperature variation and an electric field dependence of the d.c. conductivity in a carefully characterized sample of P doped Si. These results, combined with measurements of Raman scattering, far-infrared absorption and magnetic susceptibility, indicate a large characteristic electronic length (10^4 times the donor Bohr radius) at a donor concentration just below the critical density of the metal-insulator transition