  Micrometer width and nanometer thick wires with different shapes were
produced $\approx 3~\upmu$m below the surface of a diamond crystal using a
microbeam of He$^+$ ions with 1.8~MeV energy. Initial samples are amorphous and
after annealing at $T\approx 1475$~K, the wires crystallized into a
graphite-like structures, according to confocal Raman spectroscopy
measurements. The electrical resistivity at room temperature is only one order
of magnitude larger than the in-plane resistivity of highly oriented pyrolytic
bulk graphite and shows a small resistivity ratio($\rho(2{\rm K})/\rho(315{\rm
K}) \approx 1.275$). A small negative magnetoresistance below $T=200$~K was
measured and can be well understood taking spin-dependent scattering processes
into account. The used method provides the means to design and produce
millimeter to micrometer sized conducting circuits with arbitrary shape
embedded in a diamond matrix.
