  Majorana zero modes can appear at the wire ends of a one-dimensional
topological superconductor and manifest themselves as a quantized zero-bias
conductance peak in the tunnel spectroscopy of normal-superconductor junctions.
However, in superconductor-semiconductor hybrid nanowires, zero-bias
conductance peaks may arise owing to topologically trivial mechanisms as well,
mimicking the Majorana-induced topological peak in many aspects. In this work,
we systematically investigate the characteristics of zero-bias conductance
peaks for topological Majorana bound states, trivial quasi-Majorana bound
states and low-energy Andreev bound states arising from smooth potential
variations, and disorder-induced subgap bound states. Our focus is on the
conductance peak value (i.e., equal to, greater than, or less than $2e^2/h$),
as well as the robustness (plateau- or spike-like) against the tuning
parameters (e.g., the magnetic field and tunneling gate voltage) for zero bias
peaks arising from the different mechanisms. We find that for Majoranas and
quasi-Majoranas, the zero-bias peak values are no more than $2e^2/h$, and a
quantized conductance plateau forms generically as a function of parameters. By
contrast, for conductance peaks due to low-energy Andreev bound states or
disorder-induced bound states, the peak values may exceed $2e^2/h$, and a
conductance plateau is rarely observed unless through careful fine-tunings. Our
findings should shed light on the interpretation of experimental measurements
on the tunneling spectroscopy of normal-superconductor junctions of hybrid
Majorana nanowires.
