  To probe the quenching mechanisms under high current densities,
current-voltage curves have been measured in YBa2Cu3O7-delta thin film
microbridges with widths lower than the thermal diffusion length. This
condition was obtained by using microbridge widths under 100 micrometers and
stepped ramps of one millisecond step duration. Whereas the flux-flow
resistivity is found to be microbridge-width independent, strong width
dependence of the quenching current density is observed. These results provide
a direct experimental demonstration that for high current densities varying in
the millisecond range the transition to a highly dissipative state is due to
self heating driven by "conventional" (non-singular) flux flow effects.
