  We discuss pairing signatures in mesoscopic nanowires with variable
attractive pairing interaction. Depending on wire length, density, and
interaction strength, these systems realize a simultaneous bulk-to-mesoscopic
and BCS-BEC crossover, which we describe in terms of the parity parameter that
quantifies the odd-even energy difference and generalizes the bulk Cooper pair
binding energy to mesoscopic systems. We show that the parity parameter can be
extracted from recent measurements of conductance oscillations in SrTiO$_3$
nanowires by G. Cheng et al. [Nature 521, 196 (2015)], where it marks the
critical magnetic field that separates pair and single-particle currents. Our
results place the experiment in the fluctuation-dominated mesoscopic regime on
the BCS side of the crossover.
