We report an experimental observation of impurity-induced conductance dips in quantized channels as predicted by previous theoretical studies. Our experiments use quantum point contacts on a two-dimensional electron gas in a modulation-doped GaAs/AlxGa1-xAs heterostructure. The electron gas has a sheet density of 1.2×1011 cm-2 and a mobility of 4.6×105 cm2/Vs measured at 50 mK. Our data, which are qualitatively very similar to those calculated with use of a two-dimensional Anderson model, strongly suggest that we are observing both the erosion of conductance quantization, and localization in the presence of an impurity-induced random potential. © 1990 The American Physical Society.