Quantized conductance of a suspended graphene nanoconstriction

One of the most promising characteristics of graphene(1) is the ability of charge carriers to travel through it ballistically over hundreds of nanometres. Recent developments in the preparation of high mobility graphene(2-4) should make it possible to study the effects of quantum confinement in graphene nanostructures in the ballistic regime. Of particular interest are those effects that arise from edge states, such as spin polarization at zigzag edges(5) of graphene nanoribbons(6,7) and the use of graphene's valley-degeneracy for 'valleytronics'(8). Here we present the observation of quantized conductance(9,10) at integer multiples of 2e(2)/h at zero magnetic field in a high mobility suspended graphene ballistic nanoconstriction. This quantization evolves into the typical quantum Hall effect for graphene at magnetic fields above 60mT. Voltage bias spectroscopy reveals an energy spacing of 8meV between the first two subbands. A pronounced feature at 0.6 x 2e(2)/h present at a magnetic field as low as similar to 0.2 T resembles the '0.7 anomaly' observed in quantum point contacts in a GaAs-AlGaAs two-dimensional electron gas, possibly caused by electron-electron interactions(11).