  We have studied magnetotransport properties of a high-mobility
two-dimensional electron system subject to weak electric fields. At low
magnetic field $B$, the differential resistivity acquires a correction $\delta
r \propto -\lambda^2 j^2/B^2$, where $\lambda$ is the Dingle factor and $j$ is
the current density, in agreement with theoretical predictions. At higher
magnetic fields, however, $\delta r$ becomes $B$-independent, $\delta r \propto
-j^2$. While the observed change in behavior can be attributed to a crossover
from overlapping to separated Landau levels, full understanding of this
behavior remains a subject of future theories.
