Activate input mask for list field columns. If no other field types have input masks enabled you will also need to enqueue the masked input script, see the following gist for an example https://gist.github.com/richardW8k/7529522

ListFieldMaskedInput

<script>
function ListFieldMaskedInput( formId, fieldId, column, mask ){
    jQuery( '.gfield_list_' + fieldId + '_cell' + column + ' input' ).mask( mask );
    jQuery( '#field_' + formId + '_' + fieldId ).on( 'click', '.add_list_item', function(){ 
        jQuery( '.gfield_list_' + fieldId + '_cell' + column + ' input' ).mask( mask ) 
    });
}
ListFieldMaskedInput( 11, 2, 1, '9999' );
/**
 * Add a function call for each list field column you want to activate an input mask for
 * ListFieldMaskedInput( formId, fieldId, column, 'mask' );
 * for examples of input masks see http://www.gravityhelp.com/documentation/page/Input_Mask
 */
</script>