  We use continuous-time quantum Monte Carlo simulations to study retardation
effects in the metallic, quarter-filled Holstein model in one dimension. Based
on results which include the one- and two-particle spectral functions as well
as the optical conductivity, we conclude that with increasing phonon frequency
the ground state evolves from one with dominant diagonal order---2k_F charge
correlations---to one with dominant off-diagonal fluctuations, namely s-wave
pairing correlations. In the parameter range of this crossover, our numerical
results support the existence of a spin gap for all phonon frequencies. The
crossover can hence be interpreted in terms of preformed pairs corresponding to
bipolarons, which are essentially localised in the Peierls phase, and
"condense" with increasing phonon frequency to generate dominant pairing
correlations.
