A magnetoresistive sensor includes first and second magnetic leads and a constricted junction joining the first and second magnetic leads. The first and second magnetic leads are formed of a magnetic and electrically conductive material. The constricted junction includes a junction core formed of magnetic and electrically conductive material. In one embodiment, the constricted junction also includes an ion implanted outer shell portion that at least partially surrounds the junction core and has reduced magnetic and electrical conductivity relative to the junction core. In another embodiment, the junction core has a length that is defined by a distance separating the first and second magnetic leads, and a width that is perpendicular to the length and is substantially less than an average unrestricted magnetic domain wall width corresponding to the magnetic material of the junction core.