  We employed a scanning Hall probe microscope to detect the hopping of
individual vortices between pinning sites along grain boundaries in YBCO thin
films in the presence of an applied current. Detecting the motion of individual
vortices allowed us to probe the current-voltage (I-V) characteristics of the
grain boundary with voltage sensitivity below a femto-volt. We find a very
sharp onset of dissipation with V~I^n with an unprecedented high exponent of
n~290 that shows essentially no dependence on temperature or grain boundary
angle. Our data have no straightforward explanation within the existing grain
boundary transport models.
