  A two-dimensional topological insulator (2DTI) is guaranteed to have a
helical 1D edge mode in which spin is locked to momentum, producing the quantum
spin Hall effect and prohibiting elastic backscattering at zero magnetic field.
No monolayer material has yet been shown to be a 2DTI, but recently the Weyl
semimetal WTe2 was predicted to become a 2DTI in monolayer form if a bulk gap
opens. Here, we report that at temperatures below about 100 K monolayer WTe2
does become insulating in its interior, while the edges still conduct. The edge
conduction is strongly suppressed by in-plane magnetic field and is independent
of gate voltage, save for mesoscopic fluctuations that grow on cooling due to a
zero-bias anomaly which reduces the linear-response conductance. Bilayer WTe2
also becomes insulating at low temperatures but does not show edge conduction.
Many of these observations are consistent with monolayer WTe2 being a 2DTI.
However, the low temperature edge conductance, for contacts spacings down to
150 nm, is below the quantized value,
