  The temperature and density dependence of the phase coherence time
$\tau_\phi$ in high-mobility silicon inversion layers was determined from the
magnetoresistivity due to weak localization. The upper temperature limit for
single-electron quantum interference effects was delineated by comparing
$\tau_\phi$ with the momentum relaxation time $\tau$. A comparison between the
density dependence of the borders for quantum interference effects and the
strong resistivity drop reveals that theses effects are not related to each
other. As the strong resistivity drop occurs in the Drude regime, the apparent
metallic behavior can not be caused by quantum coherent effects.
