  In the quantum anomalous Hall effect, quantized Hall resistance and vanishing
longitudinal resistivity are predicted to result from the presence of
dissipationless, chiral edge states and an insulating 2D bulk, without
requiring an external magnetic field. Here, we explore the potential of this
effect in magnetic topological insulator thin films for metrological
applications. Using a cryogenic current comparator system, we measure
quantization of the Hall resistance to within one part per million and
longitudinal resistivity under 10 m$\Omega$ per square at zero magnetic field.
Increasing the current density past a critical value leads to a breakdown of
the quantized, low-dissipation state, which we attribute to electron heating in
bulk current flow. We further investigate the pre-breakdown regime by measuring
transport dependence on temperature, current, and geometry, and find evidence
for bulk dissipation, including thermal activation and possible variable-range
hopping.
