  The electrical resistivity rho_c of the underdoped cuprate superconductor
YBCO was measured perpendicular to the CuO_2 planes on ultra-high quality
single crystals in magnetic fields large enough to suppress superconductivity.
The incoherent insulating-like behavior of rho_c at high temperature,
characteristic of all underdoped cuprates, is found to cross over to a coherent
regime of metallic behavior at low temperature. This crossover coincides with
the emergence of the small electron pocket detected in the Fermi surface of
YBCO via quantum oscillations, the Hall and Seebeck coefficients and with the
detection of a unidirectional modulation of the charge density as seen by
high-field NMR measurements. The low coherence temperature is quantitatively
consistent with the small hopping integral t_perp inferred from the splitting
of the quantum oscillation frequencies. We conclude that the Fermi-surface
reconstruction in YBCO at dopings from p = 0.08 to at least p = 0.15,
attributed to stripe order, produces a metallic state with 3D coherence deep in
the underdoped regime.
