add_action() is not working in wordpress?

I create a function to receive the ajax calls in function.php (wordpress)

add_action( 'wp_ajax_contact_us_receiver', 'contact_us_receiver_callback' );
function contact_us_receiver_callback() {
    echo "you are at right place ";
}

calling it in ajax url -:

              post_data = $("#contact_us_form").serialize();
                //Ajax post data to server
                $.post('contact_us_receiver', post_data, function(response){  

                    //load json data from server and output message     
                    if(response.type == 'error')
                    {
                     output = '<div class="alert alert-danger">'+response.text+'</div>';
                    }else{
                        output = '<div class=" alert alert-success">'+response.text+'</div>';
                        $('#contact_us_form input ').val('');
                         $('#contact_us_form select ').val('');
                      $('#contact_us_form textarea ').val('');
                    }

                    $('#result').html(output);
                    $('#contact_us_form input ').css('border','');
                    $('#contact_us_form select ').css('border','');
                }, 'json');

But it is giving me 404 not found error in console . please advise me how can i make call to my function .