  We report magnetoconductivity measurements on YBa_2Cu_3O_x (x=6.25 and 6.36)
single crystals. Our main result is that both the in-plane Delta_sigma_ab and
out-of-plane Delta_sigma_c magnetoconductivities exhibit the field dependence
characteristic of "two-dimensional" quantum interference in applied magnetic
fields H parallel to the c-axis. Namely, Delta_sigma_c,ab are proportional to
ln(H/H_0), with Delta_sigma_c/sigma_c substantially greater than
Delta_sigma_ab/sigma_ab. We interpret this result as evidence of interlayer
incoherence in these crystals, so that the phase-coherent trajectories are
mostly confined to one bilayer.
