Phonon-Limited Drift of the Electron Bubble in Superfluid Helium

We present measurements of the negative ion mobility in He II for $T<~0.5$°K. We show that the usual kinetic-theory approach to ion drift in He II is inadequate, and we describe a more realistic model. It is found that inelastic phonon scattering is the dominant process determining the mobility of the electron bubble at these temperatures.