Mobility of 1st group elements in presence of electric field decrease down the group .As we can clearly see from the formula of mobility mu= -e tau/m. Mobility is directly proportional to tau(relaxation time). As we move down the group size increase it means nuclear force is decrease due to which the electrons of elements is found some easiness to collide and hence relaxations time decrease due to which mobility is also decrease. It means mobility of 1st member of group 1st is largest and mobility of last member is smallest.