  It has been suggested that all resistive-switching memory cells are
memristors. The latter are hypothetical, ideal devices whose resistance, as
originally formulated, depends only on the net charge that traverses them.
Recently, an unambiguous test has been proposed [J. Phys. D: Appl. Phys. {\bf
52}, 01LT01 (2019)] to determine whether a given physical system is indeed a
memristor or not. Here, we experimentally apply such a test to both in-house
fabricated Cu-SiO2 and commercially available electrochemical metallization
cells. Our results unambiguously show that electrochemical metallization memory
cells are not memristors. Since the particular resistance-switching memories
employed in our study share similar features with many other memory cells, our
findings refute the claim that all resistance-switching memories are
memristors. They also cast doubts on the existence of ideal memristors as
actual physical devices that can be fabricated experimentally. Our results then
lead us to formulate two memristor impossibility conjectures regarding the
impossibility of building a model of physical resistance-switching memories
based on the memristor model.
