  A yet unexplored area in graphene electronics is the field of quantum
ballistic transport through graphene nanostructures. Recent developments in the
preparation of high mobility graphene are expected to lead to the experimental
verification and/or discovery of many new quantum mechanical effects in this
field. Examples are effects due to specific graphene edges, such as spin
polarization at zigzag edges of a graphene nanoribbon and the use of the valley
degree of freedom in the field of graphene valleytronics8. As a first step in
this direction we present the observation of quantized conductance at integer
multiples of 2e^2/h at zero magnetic field and 4.2 K temperature 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 8 meV
between the first two subbands. A pronounced feature at 0.6 2e^2/h present at a
magnetic field as low as ~0.2T resembles the "0.7 anomaly" observed in quantum
point contacts in a GaAs-AlGaAs two dimensional electron gas, having a possible
origin in electron-electron interactions.
