  We have studied an electron transport in inversion layers of high-mobility
Si(100) samples. At high electron concentrations and temperatures below 4.2 K,
two series of Shubnikov-de Haas oscillations have been observed. The
temperature damping of the second series oscillations indicates that the second
occupied subband belongs to the first energy level of the fourfold-degenerate
ladder $0'$. Samples with two occupied subbans exhibit a strong anomalous
negative magnetoresitance, reaching $\rm \approx 25 \%$ of a zero field value
at $B =$ 12 T. The resistance decrease is more pronounced for lower
temperatures and higher electron concentrations. We explain this behaviour by
an increase of the second subband mobility due to the freezing-out of the
scattering of $0'$ electrons. Based on the measured periods of SdH
oscillations, we conclude that the electrons are distributed inhomogeneously
beneath the sample gate.
