What element has good conductivity but is also brittle?
Whether a "metalloid" is both brittle and a good conductor depends on what you mean by "metalloid", which varies in literature, what is a "good" level of (electrical) conductivity, and even what exactly you mean by "brittle" versus "ductile".  Silicon, for instance, is only a semiconductor, with much less electrical conductivity than metals, when pure.

Wikipedia identifies beryllium and magnesium as brittle metals, although the latter can be made ductile with minor alloying.  Zinc is called "slightly brittle".