  We report measurements of the electric field vs. current density [E(J]
characteristics in the mixed state of amorphous Nb0.7Ge0.3 microbridges. Close
to the transition temperature Tc the Larkin-Ovchinnikov theory of nonlinear
flux flow and the related instability describes the data quantitatively up to ~
70% of the upper critical magnetic field Bc2 and over a wide electric field
range. At lower temperatures the nonlinearities of E(J)can be described by
electron heating which reduces Bc2 and leads to a second type of flux flow
instability, as shown by a scaling analysis of the high-dissipation data.
