ON MOBILITY OF SURFACE ELECTRONS IN LIQUID HELIUM AT TEMPERATURES UP TO 20 mK B.N. Eselson, A.S. Rybalko, Yu.Z. Kovdrya, A.A. Golub and S.S. Sokolov J. Phys. Colloques, 39 C6 (1978) C6-338 DOI: 10.1051/jphyscol:19786150