Energy of electron states of the double quantum well wire (solid lines) where the barrier b = r_b − r_a varies, while r_a and d = r_c − r_b are maintaining fixed (a) r_a = 6.0 nm and d = 6.0 nm, (b) r_a = 6.0 nm and d = 8.0 nm, (c) r_a = 8.0 nm and d = 6.0 nm. The dotted lines correspond to a quantum well wire of cylindrical ring geometry with width d, while dashed lines correspond to the energies of a simple quantum well wire with radius r_a.