  We study elastic collisions in an optically trapped spin mixture of fermionic
lithium atoms in the presence of magnetic fields up to 1.5kG by measuring
evaporative loss. Our experiments confirm the expected magnetic tunability of
the scattering length by showing the main features of elastic scattering
according to recent calculations. We measure the zero crossing of the
scattering length that is associated with a predicted Feshbach resonance at
530(3)G. Beyond the resonance we observe the expected large cross section in
the triplet scattering regime.
