  We report an experimental study of one-dimensional (1D) electronic transport
in an InSb semiconducting nanowire. Three bottom gates are used to locally
deplete the nanowire creating a ballistic quantum point contact with only a few
conducting channels. In a magnetic field, the Zeeman splitting of the
corresponding 1D subbands is revealed by the emergence of conductance plateaus
at multiples of $e^2$/h, yet we find a quantized conductance pattern largely
dependent on the configuration of voltages applied to the bottom gates. In
particular, we can make the first plateau disappear leaving a first conductance
step of 2$e^2/h$, which is indicative of a remarkable two-fold subband
degeneracy that can persist up to several Tesla. For certain gate voltage
settings, we also observe the presence of discrete resonant states producing
conductance features that can resemble those expected from the opening of a
helical gap in the subband structure. We explain our experimental findings
through the formation of two spatially separated 1D conduction channels.
