  For insulating Si:B with dopant concentrations from 0.75 n_c to the critical
concentration n_c, the conductivity ranging over five orders of magnitude
collapses using a single scaling parameter T* onto a universal curve of the
form $\sigma (T) =\sigma_0 f (T^*/T)$ with a temperature-independent prefactor
of the order of Mott's minimum metallic conductivity. The function $f (T^*/T) =
e^{-(T^*/T)^{\beta}}$ with \beta = 1/2 when T^*/T>10, corresponding to
Efros-Shklovskii variable-range hopping. For T^*/T < 8 the exponent \beta =
1/3, a value expected for Mott variable-range hopping in two rather than three
dimensions. The temperature-independent prefactor implies hopping that is not
mediated by phonons.
