  We first present detailed measurements of the rounding behavior around the
superconducting transition temperature, $T_c$, of the in-plane electrical
conductivity, magnetoconductivity and magnetization, including the low and
moderate magnetic field regimes, in a high-quality single crystal and a thin
film of the prototypical optimally-doped YBa$_2$Cu$_3$O$_{7-\delta}$ (OPT
Y-123), in which the inhomogeneity effects are minimized. Then, it is presented
a comparison of these experimental data with the phenomenological
Ginzburg-Landau (GL) approach that takes into account the unavoidable
contribution of the fluctuating pairs, the only theoretical scenario allowing
at present to analyze these roundings at the quantitative level. These analyses
demonstrate that the measured rounding effects around $T_c$ may be explained
quantitative and consistently in terms of the GL scenario, even up to the
rounding onset temperatures if the quantum localization, associated with the
shrinkage of the superconducting wavefunction, is taken into account. The
implications of our results on the pseudogap physics of optimally-doped
cuprates are also discussed.
