  The polarizability of electrons occupying the lowest subband of spatial
quantization in CdTe/Cd$_x$Hg$_{1-x}$Te/CdTe quantum wells is calculated. It is
shown that polarizability in the quantum well without cadmium is negative,
i.e., the displacement of an electron in an electric field applied
perpendicularly to the quantum well plane is opposite to the force acting on
it. The negative polarizability of 2D electrons can reduce the dielectric
constant of quantum wells by up to $(10-15)$ percent.
