  We demonstrate that the size of the electric-field-induced Rashba spin
splitting in an 80 nm wide modulation-doped InGaSb quantum well can depend
strongly on the spatial variation of the electric field. In a slightly
asymmetric quantum well it can be an order of magnitude stronger than for the
average uniform electric field. For even smaller asymmetry spin subbands can
have wave functions and/or expectation values of the spin direction that are
completely changed as the in-plane wave vector varies. The Dresselhaus effect
can give an anticrossing at which the spin rapidly flips.
