  We report electronic measurements on high-quality graphene nanoconstrictions
(GNCs) fabricated in a transmission electron microscope (TEM), and the first
measurements on GNC conductance with an accurate measurement of constriction
width down to 1 nm. To create the GNCs, freely-suspended graphene ribbons were
fabricated using few-layer graphene grown by chemical vapor deposition. The
ribbons were loaded into the TEM, and a current-annealing procedure was used to
clean the material and improve its electronic characteristics. The TEM beam was
then used to sculpt GNCs to a series of desired widths in the range 1 - 700 nm;
after each sculpting step, the sample was imaged by TEM and its electronic
properties measured in-situ. GNC conductance was found to be remarkably high,
comparable to that of exfoliated graphene samples of similar size. The GNC
conductance varied with width approximately as, where w is the constriction
width in nanometers. GNCs support current densities greater than 120 \muA/nm2,
two orders of magnitude higher than has been previously reported for graphene
nanoribbons and 2000 times higher than copper.
