  We observe dark-state polariton collapses and revivals in a quantum memory
based on electromagnetically induced transparency on a cloud of cold cesium
atoms in a magnetic field. Using $\sigma^+$ polarized signal and control beams
in the direction of the magnetic field, we suppress the dark-state polariton
collapses by polarizing the atoms towards one of the stretched Zeeman states
and optimizing the frequency detuning of the control beam. In this way, we
demonstrate a quantum memory with only partial dark-state polariton collapses,
making the memory usable at any storage time, not only at discretized times of
revivals. We obtain storage time of more than 400 $\rm{\mu}$s, which is ten
times longer than what we can achieve by trying to annul the magnetic field.
