  We report on a zero magnetic field transport study of a two-dimensional,
variable-density, hole system in GaAs. As the density is varied we observe, for
the first time in GaAs-based materials, a crossover from an insulating behavior
at low-density, to a metallic-like behavior at high-density, where the metallic
behavior is characterized by a large drop in the resistivity as the temperature
is lowered. These results are in agreement with recent experiments on Si-based
two-dimensional systems by Kravchenko et al. and others. We show that, in the
metallic region, the resistivity is dominated by an exponential
temperature-dependence with a characteristic temperature which is proportional
to the hole density, and appear to reach a constant value at lower
temperatures.
