  We derive closed expressions for the nuclear breakup cross sections in the
adiabatic limit using the Austern-Blair theory. These expressions are
appropriate for the breakup of weakly bound nuclei. The concept of an exit
doorway that mediates the coupling between the entrance channel and the breakup
continuum is used. We prove the validity of the scaling law that dictates that
the nuclear breakup cross section scales linearly with the radius of the
target. We also compare our results for the nuclear breakup cross section of
$^{11}$Be, $^8$B on several targets with recent CDCC calculation.
