Magnetoconductivity due to quantum interference in strongly underdoped YBa 2 Cu 3 O x

We report magnetoconductivity measurements on YBa{sub 2}Cu{sub 3}O{sub x} (x=6.25 and 6.36) single crystals. Our main result is that both the in-plane {Delta}{sigma}{sub ab} and out-of-plane {Delta}{sigma}{sub c} magnetoconductivities exhibit the field dependence characteristic of ''two-dimensional'' quantum interference in applied magnetic fields H||c. Namely, {Delta}{sigma}{sub c,ab}{proportional_to}lnH/H{sub 0}>0, with {Delta}{sigma}{sub c}/{sigma}{sub c} substantially greater than {Delta}{sigma}{sub ab}/{sigma}{sub ab}. We interpret this result as an evidence of interlayer incoherence in these crystals, so that the phase-coherent trajectories are mostly confined to one bilayer.