  Using a linear array of voltage electrodes with a separation of several
micrometers on a $20 $nm thick and 30 $\mu$m long multigraphene sample we show
that the measured resistance does not follow the usual length dependence
according to Ohm's law. The deviations can be quantitatively explained taking
into account Sharvin-Knudsen formula for ballistic transport. This allows us to
obtain without free parameters the mean free path of the carriers in the sample
at different temperatures. In agreement with recently reported values obtained
with a different experimental method, we obtain that the carrier mean free path
is of the order of $\sim 2 \mu$m with a mobility $\mu \sim 10^7
$cm$^{2}$V$^{-1}$s$^{-1}$. The results indicate that the usual Ohm's law is not
adequate to calculate the absolute resistivity of mesoscopic graphite samples.
