  We report on nonlinear transport measurements in a two-dimensional electron
gas hosted in GaAs/AlGaAs heterostructures. Upon application of direct current,
the low-temperature differential resistivity acquires a positive correction,
which exhibits a pronounced maximum followed by a plateau. With increasing
temperature, the nonlinearity diminishes and disappears. These observations can
be understood in terms of a crossover from the Bloch-Gr\"uneisen regime to the
quasielastic scattering regime as the electrons are heated by direct current.
Calculations considering the interaction of electrons with acoustic phonons
provide a reasonable description of our experimental findings.
