  The radial confining potential in a semiconductor nanowire plays a key role
in determining its quantum transport properties. Previous reports have shown
that an axial magnetic field induces flux-periodic conductance oscillations
when the electronic states are confined to a shell. This effect is due to the
coupling of orbital angular momentum to the magnetic flux. Here, we perform
calculations of the energy level structure, and consequently the conductance,
for more general cases ranging from a flat potential to strong surface band
bending. The transverse states are not confined to a shell, but are distributed
across the nanowire. It is found that, in general, the subband energy spectrum
is aperiodic as a function of both gate voltage and magnetic field. In
principle, this allows for precise identification of the occupied subbands from
the magnetoconductance patterns of quasi-ballistic devices. The aperiodicity
becomes more apparent as the potential flattens. A quantitative method is
introduced for matching features in the conductance data to the subband
structure resulting from a particular radial potential, where a functional form
for the potential is used that depends on two free parameters. Finally, a
short-channel InAs nanowire FET device is measured at low temperature in search
of conductance features that reveal the subband structure. Features are
identified and shown to be consistent with three specific subbands. The
experiment is analyzed in the context of the weak localization regime, however,
we find that the subband effects predicted for ballistic transport should
remain visible when back scattering dominates over interband scattering, as is
expected for this device.
