  In this article we report on the experimental realization of dimensional
crossover phenomena in the chain compound PrBa$_2$Cu$_4$O$_8$ using
temperature, high magnetic fields and disorder as independent tuning
parameters. In purer crystals of PrBa$_2$Cu$_4$O$_8$, a highly anisotropic
three-dimensional Fermi-liquid state develops at low temperatures. This
metallic state is extremely susceptible to disorder however and localization
rapidly sets in. We show, through quantitative comparison of the relevant
energy scales, that this metal/insulator crossover occurs precisely when the
scattering rate within the chain exceeds the interchain hopping rate(s), i.e.
once carriers become confined to a single conducting element.
