We use Huckel theory to examine interference effects on conductance of a wire when a 'lollypop' side-chain is bonded to it, acting as a resonance cavity, A clear signature of interference is found at these ballistic conducting systems, stronger in small systems. Gating effects are enhanced by the presence of the loop, where the electronic wavefunctions can experience large changes in phase. Using an 'interference index', I = mod(S, 2) + mod(L, 4), where S, L are stick and loop lengths, respectively, we conclude that interference is constructive (destructive) and conductance high (low) when I = 0,4(I = 2). (C) 2004 Elsevier B.V. All rights reserved.