  The production of antiprotons in $p+A$ reactions is calculated in a
microscopic transport approach employing hadronic and string degrees of freedom
(HSD). It is found that the abundancies of antiprotons as observed by the E910
Collaboration in $p+A$ reactions at 12.3 GeV/c as well as 17.5 GeV/c can
approximately be described on the basis of primary proton-nucleon and secondary
meson-baryon production channels for all targets. The transport calculations
demonstrate that the antiproton rapidity distributions for heavy targets are
sensitive to the $\bar{p}$ (or hadron) formation time in the nuclear medium.
Within our analysis the data from the E910 Collaboration are reasonably
described with a formation time of $ 0.4-0.8$ fm/c in the hadron rest frame.
