Kohn Effect in Na and other Metals

It is pointed out that if the Kohn effect is looked for, not as an obvious kink in the phonon dispersion curves of metals as originally suggested, but as a damped sinusoidal variation of the interplanar force constants with separation, then (1) the effect has been observed in many metals and (2) for Na, which is very close to a free-electron metal, the measured force constants oscillate with the expected wavelength ($\frac{\pi }{k_F}$). From the data for Al and Pb, it would appear that even though the Fermi surface is quite close to a free-electron sphere in the extended zone, electrons do not scatter isotropically around this surface, but presumably stay within their respective Brillouin zones even at temperatures of the order of the Debye $\theta$.