The answer to the existence question is certainly yes, at least for “trunk size” devices. Unclassified sources have reported that small nuclear devices were developed by both the United States and the Soviet Union during the Cold War, a direct result of the work on tactical nuclear weapons. These would not fit in a briefcase, but are portable by one or two people.
The U.S. developed a class of devices called “Atomic Demolition Munitions” (ADM), intended for use as atomic land mines. ADMs were in the U.S. inventory from the late 1950’s until such weapons were phased out by arms-control agreements in the 1980’s. A version of the ADM for use by Special Forces, the “Special Atomic Demolition Munitions” (SADM), was suitcase or duffle bag size, weighing less than 100 pounds (photo, left, is SADM packing case). The top photo on this page is from a declassified film showing a demonstration of the SADM in the late 1960s. The exact status of these weapons today is unclear.
The Soviet Union’s small nuclear devices were developed for nuclear mines and possibly for Spetsnaz attacks (Special Forces). In 1997, General Aleksandr Lebed claimed that the Soviet Union created one hundred and fifteen atomic demolition munitions (ADMs), low-yield, one kiloton devices that were small, portable, and without safety devices to prevent unauthorized detonation. Lebed further raised the issue of whether the ADMs were all in proper custody and accounted for. Others have contradicted Lebed — the issue is unsettled, but it is most likely that the Soviets did produce small atomic munitions, similar to the U.S. SADM.