Although colossal magnetoresistance (CMR) materials exhibit large changes in electrical resistance (up to 106%), large magnetic fields (several tesla) must be applied. To obtain a sizeable low-field effect (<102% in several millitesla), it is necessary to incorporate structural discontinuities such as grain boundaries, or other types of interfaces. The potential for applications, however, remains limited because structural discontinuities increase electrical resistance by several orders of magnitude and hence create noise. Moreover, it has proven to be difficult to fabricate structural discontinuities reproducibly. We have attempted to investigate discontinuities that are purely magnetic via transport measurements through a precisely controlled number of magnetic domain walls of known area in thin film devices of the ferromagnetic CMR perovskite La0.7Ca0.3MnO3. A sharp low-field switching seen below ∼110 K is ascribed to the formation of a precise number of magnetic domain walls, each with resistance-area product 8×10−14 Ω m2 at 77 K. This is four orders of magnitude larger than expected, suggesting that the domain walls contain an additional structure. Our findings demonstrate that CMR devices are capable of low-noise low-field switching, and suggest the possibility of exploiting a hitherto unexpected intrinsic effect reproducibly and therefore commercially. © 1999 American Institute of Physics.