Electric- and magnetic-field tuning of spin-resolved one-dimensional subbands in an InSb nanowire

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. We conclude that the subband energy spacings and g-factors are strongly affected by the tunable landscape of the local electrostatic potential. Finally, our measurements reveal the presence of discrete resonant states that, for certain gate voltage settings, produce conductance features resembling those expected from the opening of a helical gap in the subband structure.