1. A method comprising: forming a chalcogenide material between a pair of spaced electrodes such that the cross-sectional area of said chalcogenide material decreases as it extends from one electrode to the other; surrounding said chalcogenide material with at least two different materials of different coefficients of heat transfer; and providing a first material with a first heat transfer coefficient around a first chalcogenide region of smaller cross-sectional area than a second chalcogenide region and surrounding said second chalcogenide region with a second material having a heat transfer coefficient lower than said first heat transfer coefficient.