  The fluctuation spectrum responsible for the inelastic scattering in
YBa$_2$Cu$_3$O$_{6.99}$ which was recently determined from consideration of the
in-plane optical conductivity in the infrared, is used to calculate the
temperature dependence of the microwave conductivity at several measured
frequencies. Reasonable overall agreement can only be achieved if, in addition,
some impurity scattering is included within a model potential intermediate
between weak (Born) and strong (unitary) limit.
