  The quantum anomalous Hall (QAH) effect has been demonstrated in
two-dimensional topological insulator systems incorporated with ferromagnetism.
However, a comprehensive understanding of mesoscopic transport in sub-micron
QAH devices has yet been established. Here we fabricated miniaturized QAH
devices with channel widths down to 600 nm, where the QAH features are still
preserved. A back-scattering channel is formed in narrow QAH devices through
percolative hopping between 2D compressible puddles. Large resistance
fluctuations are observed in narrow devices near the coercive field, which is
associated with collective interference between intersecting paths along domain
walls when the device geometry is smaller than the phase coherence length
$L_\phi$. Through measurement of size-dependent breakdown current, we confirmed
that the chiral edge states are confined at the physical boundary with its
width on the order of Fermi wavelength.
