  Focusing on the implications of recent experiments on Majorana zero modes in
semiconductor-superconductor (SM-SC) heterostructures, we critically examine
the quantization of the zero-bias differential conductance as a possible
unambiguous signature of Majorana physics in the presence of disorder. By
numerically calculating the zero-bias conductance (ZBC) maps as function of
Zeeman splitting and chemical potential for different disorder realizations, we
show that the presence of quantized "islands" characterized by a ZBC value
(approximately) equal to $2e^2/h$ and having a finite area/volume in a
multi-dimensional parameter space represents a unique signature of Majorana
physics supporting Majorana zero modes (MZMs) or partially-separated Majorana
modes (ps-MMs). We find that in the presence of strong disorder Majorana
physics only emerges locally and gives rise to ps-MMs, which, in turn, generate
small quantized islands when one of the Majorana modes is located at the end of
the system. Observing these small islands may require sample selection and the
systematic scanning of a large volume in the control parameter space. Upon
decreasing disorder, the quantized islands increase in size and eventually
coalesce into large topological regions. Since the presence of MZMs localized
at the opposite ends of the system is typically associated with large quantized
islands, looking for MZM-induced edge-to-edge correlations is premature in the
absence of convincing experimental evidence for (even small) quantized islands.
We conclude that the observation of quantized islands demonstrates
unambiguously the presence of the key ingredients necessary for Majorana
physics, provides an excellent diagnostic tool for evaluating the disorder
strength, and, consequently, represents the next natural milestone in the
Majorana search.
