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It has been puzzling that the resistivity of high-mobility two-dimensional (2D) carrier systems in semiconductors with low carrier density often exhibits a large increase followed by a decrease when the temperature is raised above a characteristic temperature comparable with the Fermi temperature . We find that the metallic 2D hole system in a GaAs quantum well has a linear density- (-) dependent conductivity in both the degenerate () and semidegenerate () regimes. The dependence of suggests that the metallic conduction at low is associated with the increase in , the effective mobility of itinerant carriers. However, the resistivity decrease in the semidegenerate regime originates from the reduced , the density of immobile carriers in a two-phase picture.