  A weak superconducting proximity effect in the vicinity of the topological
transition of a quantum anomalous Hall system has been proposed as a venue to
realize a topological superconductor (TSC) with chiral Majorana edge modes
(CMEMs). A recent experiment [Science 357, 294 (2017)] claimed to have observed
such CMEMs in the form of a half-integer quantized conductance plateau in the
two-terminal transport measurement of a quantum anomalous Hall-superconductor
junction. Although the presence of a superconducting proximity effect
generically splits the quantum Hall transition into two phase transitions with
a gapped TSC in between, in this Rapid Communication we propose that a nearly
flat conductance plateau, similar to that expected from CMEMs, can also arise
from the percolation of quantum Hall edges well before the onset of the TSC or
at temperatures much above the TSC gap. Our Rapid Communication, therefore,
suggests that, in order to confirm the TSC, it is necessary to supplement the
observation of the half-quantized conductance plateau with a hard
superconducting gap (which is unlikely for a disordered system) from the
conductance measurements or the heat transport measurement of the transport
gap. Alternatively, the half-quantized thermal conductance would also serve as
a smoking-gun signature of the TSC.
