  Spin wave dispersion in the metallic antiferromagnet Mn$_3$Pt was
investigated just above the order-order transition temperature by using the
inelastic neutron scattering technique. The spin wave dispersion at $T = 400$ K
along [100], [110] and [111] directions was isotropic within the measurement
accuracy. The dispersion was described by $({\hbar\omega})^2 = c^2q^2 +
\Delta^2$ with $c = 190$ meV {\AA} and $\Delta = 3.3$ meV. Compared with the
dispersion at $T = 419$ K previously reported, the result demonstrates a large
reduction of the stiffness constant $c$ with increasing temperature. This is
similar to that observed in the metallic antiferromagnet FePt$_3$, and is an
indication of the itinerancy of the magnetic moments.
