Quantum Point Contacts in Monolayer WSe₂

Quantum point contacts (QPCs) have been realized in III-IV semiconductor heterostructures, and more recently in bilayer graphene and trilayer WSe₂. These QPCs are fabricated in these two-dimensional systems using electrostatic gates, and require ballistic transport from source to drain contact to observe conductance quantization. In this work, we show that it is possible to achieve high-quality QPCs on monolayer WSe₂ due to improvements in materials synthesis and contact quality. Measurements down to mK temperatures show clear conductance quantization over micron-sized source-drain length scales. In WSe₂, at zero field, a two-fold degeneracy is expected at the top of the valence band due to the presence of strong spin-orbit interactions. Surprisingly, we find that the observed conductance plateaux are quantized in units of e²/h, indicating that the spin-valley degeneracy is lifted even without the application of magnetic fields. Further, the first plateau has a systematic dependence on charge carrier density and on applied magnetic field, in a manner similar to the "0.7-anomaly" reported in previous experiments on III-V semiconductors. We will discuss the nature of the anomaly seen here and the relevance of spin-orbit coupling to transport through this system.