From Ohmic to Ballistic Transport in Oriented Graphite
  In this work we show that for a quasi-2D system of size $\Omega$ and thickness $t$ the resistance goes as $(2\rho/\pi t)\ln(\Omega/W)$, diverging logarithmically with the size. Measurements in highly oriented pyrolytic graphite (HOPG) as well as numerical simulations confirm this relation. Furthermore, we present an experimental method that allows us to obtain the carriers mean free path $l(T)$, the Fermi wavelength $\lambda(T)$ and the mobility $\mu(T)$ directly from experiments without adjustable parameters. Measuring the electrical resistance through microfabricated constrictions in HOPG and observing the transition from ohmic to ballistic regime we obtain that $0.2 \mu$m $\lesssim l \lesssim 10 \mu$m, $0.1 \mu$m $\lesssim \lambda \lesssim 2 \mu$m and a mobility $5 \times 10^4$ cm$^2$/Vs $ \lesssim \mu \lesssim 4 \times 10^7$ cm$^2$/Vs when the temperature decreases from 270K to 3K. A comparison of these results with those from literature indicates that conventional, multiband Boltzmann-Drude approaches are inadequate for oriented graphite. The upper value obtained for the mobility is much larger than the mobility graphene samples of micrometer size can have. 
N. Garc\'ia, P. Esquinazi, J. Barzola-Quiquia, B. Ming and D. Spoddig