  The low-temperature conductivity of low-density, high-mobility,
two-dimensional hole systems in GaAs was studied. We explicitly show that the
metal-insulator transition, observed in these systems, is characterized by a
well-defined critical density, p_0c. We also observe that the low-temperature
conductivity of these systems depends linearly on the hole density, over a wide
density range. The high-density linear conductivity extrapolates to zero at a
density close to the critical density.
