  The quantum anomalous Hall effect (QAHE) realizes dissipationless
longitudinal resistivity and quantized Hall resistance without the need of an
external magnetic field. However, when reducing the device dimensions or
increasing the current density, an abrupt breakdown of the dissipationless
state occurs with a relatively small critical current, limiting the
applications of the QAHE. We investigate the mechanism of this breakdown by
studying multi-terminal devices and identified that the electric field created
between opposing chiral edge states lies at the origin. We propose that
electric-field-driven percolation of two-dimensional charge puddles in the
gapped surface states of compensated topological-insulator films is the most
likely cause of the breakdown.
