  We study the quantization of Dirac fermions in lithographically defined
graphene nanoconstrictions. We observe quantized conductance in single
nanoconstrictions fabricated on top of a thin hexamethyldisilazane layer over a
Si/SiO_2 wafer. This nanofabrication method allows us to obtain well defined
edges in the nanoconstrictions, thus reducing the effects of edge roughness on
the conductance. We prove the occurrence of ballistic transport and identify
several size quantization plateaus in the conductance at low temperature.
Experimental data and numerical simulations show good agreement, demonstrating
that the smoothing of the plateaus is not related to edge roughness but to
quantum interference effects.
