  We measure and analyze the resistivity of $Bi_2Sr_2CaCu_2O_{8+\delta}$
crystals for different doping $\delta$. We obtain the fraction of carrier
$\eta(T,\delta) = n_g/n_{TOT}$ that do not participate to the c-axis
conductivity. All the curves $\eta(T,\delta)$ collapse onto a universal curve
when plotted against a reduced temperature
$x=[T-\Theta(\delta)]/\Delta^{*}(\delta)$. We find that at the superconducting
transition $n_g$ is doping independent. We also show that a magnetic field up
to 14 T does not affect the degree of localization in the (a,b) planes but
widens the temperature range of the x-scaling by suppressing the
superconducting phase coherence.
