  Doping a topological insulator (TI) film with transition metal ions can break
its time-reversal symmetry and lead to the realization of the quantum anomalous
Hall (QAH) effect. Prior studies have shown that the longitudinal resistance of
the QAH samples usually does not vanish when the Hall resistance shows a good
quantization. This has been interpreted as a result of the presence of possible
dissipative conducting channels in magnetic TI samples. By studying the
temperature- and magnetic field-dependence of the magnetoresistance of a
magnetic TI sandwich heterostructure device, we demonstrate that the
predominant dissipation mechanism in thick QAH insulators can switch between
non-chiral edge states and residual bulk states in different magnetic field
regimes. The interactions between bulk states, chiral edge states, and
non-chiral edge states are also investigated. Our study provides a way to
distinguish between the dissipation arising from the residual bulk states and
non-chiral edge states, which is crucial for achieving true dissipationless
transport in QAH insulators and for providing deeper insights into QAH-related
phenomena.
