  The integer and fractional quantum Hall states are known to break down at
high dc bias, exhibiting deviation from the ideal incompressible behavior. We
measure breakdown of the \nu = 2, 3, 4, 5 integer and the \nu = 4/3 and 5/3
fractional states in a quantum point contact (QPC) of lithographic width ~600
nm. Dependence of the critical current on magnetic field, QPC gate voltage, and
QPC width are presented. Of particular interest, the critical current of the
4/3 and 5/3 fractional states shows the opposite dependence on QPC width
compared to the integer states. This previously unobserved result is not
explained by current theories of breakdown.
