  The single-particle structure of the $N=27$ isotones provides insights into
the shell evolution of neutron-rich nuclei from the doubly-magic $^{48}$Ca
toward the drip line. $^{43}$S was studied employing the one-neutron knockout
reaction from a radioactive $^{44}$S beam. Using a combination of prompt and
delayed $\gamma$-ray spectroscopy the level structure of $^{43}$S was
clarified. Momentum distributions were analyzed and allowed for spin and parity
assignments. The deduced spectroscopic factors show that the $^{44}$S
ground-state configuration has a strong intruder component. The results were
confronted with shell model calculations using two effective interactions.
General agreement was found between the calculations, but strong population of
states originating from the removal of neutrons from the $2p_{3/2}$ orbital in
the experiment indicates that the breakdown of the $N=28$ magic number is more
rapid than the theoretical calculations suggest.
