  We have examined data from pulsars B0950+08 and B0329+54 for evidence of
temporally coherent radiation using the modified coherence function (MCF)
technique of Jenet et al. (2001). We consider the influence of both
instrumental bandpass and interstellar propagation effects. Even after removal
of the effects due to the instrumental bandpass, we detect a signature in the
MCF of our PSR B0329+54 data which is consistent with the definition of a
coherent signal. However, we model the effects due to interstellar
scintillation for this pulsar and show that it reproduces the observed
signature. In particular, the temporal coherence time is close to the
reciprocal of the decorrelation bandwidth due to diffractive scintillation.
Furthermore, comparison of the coherence times of three pulsars reported by
Jenet et al. (2001) with their expected diffractive decorrelation bandwidths
suggests that the detection of coherence in these pulsars is also likely a
result of interstellar scintillation, and is not intrinsic to the pulsars.
