Fig. 3

Voltage bias spectroscopy of the helical gap. a Conductance measurement as a function of QPC gate V _g and source-drain bias voltage V _sd at B = 4 T. The observed helical gap is a stable feature in voltage bias. Dotted lines are drawn as guide to the eye indicating the linecut presented in b (red), as well as the edges of the 0.5·G ₀ plateau (purple), helical gap (green) and the 1·G ₀ and 1.5·G ₀ plateaus (black). b, Linecut of a taken at V _sd = 0 mV together with the color scheme explained in Fig. 1. c Evolution of the energy levels extracted from the scans similar to a, at increasing magnetic field. Fits with intercept fixed at zero (dotted lines) give the g-factor of the first subband and the offset angle via \(g = 1{\rm{/}}\left( {{\mu _{\rm{B}}}{\kern 1pt} \cos \theta } \right) \cdot {\rm{d}}E{\rm{/d}}B\) and \({E_{{\rm{helical}}}}/{E_{{\rm{Zeeman}}}} \approx \tan \theta \). We find g = 38 ± 1 and θ = 13° ± 2°. Individual scans are shown in Supplementary Fig. 2