On moving down the group, the atomic and ionic size of alkali metals increases. As the size increases, the extent of hydration decreases in aqueous solution. The electrical conductivity or ionic mobility is inversely proportional to extent of hydration. So, extent of hydration Li+>Na+>K+>Cs+. Hence, the increasing order of electrical conductivity of alkali metals ions in their aqueous solution is Cs+>K+>Na+>Li+