  A low-field spin flop transition in the quasi one-dimensional antiferromagnet
\ba is exploited to study the polarization dependence of low-energy magnetic
excitations. The measured longitudinal spectrum is best described as single
broad continuum, with no sharp ``longitudinal mode'', in apparent contradiction
with the commonly used chain-MF/RPA theories. The observed behavior is also
quite different than that previously seen in the related \kcuf material,
presumably due to a large difference in the relative strength of inter-chain
interactions. The results highlight the limitations of the chain-MF/RPA
approach.
