how to call knockout function in nested observable array

I have a brand list and brands contains car list. Declared simple view model in knockout on http://jsfiddle.net/PZqEk/1/ . this.cars is undefined in removeCar function when clicking remove button. How to delete clicked car with best practice?

Html:

<h1 data-bind="text:title"></h1>
<table>
    <tbody data-bind="foreach: brands">
        <tr>
            <td>
                <span data-bind="text: nameOfBrand"></span>
                <ul data-bind="foreach: cars">
                    <li>
                        <span data-bind="text: nameOfCar"></span>
                        (<span data-bind="text: yearOfCar"></span>)
                        <input data-bind="click: $root.removeCar" type="button" value="remove"/>
                    </li>
                </ul>
            </td>
        </tr>
    </tbody>
</table>

Javascript:

function RootModel()
{
    this.title = ko.observable("Dummy Title");
    this.brands = ko.observableArray();

    var b1 = new brandModel("Audi");
    b1.cars.push(new carModel("A3", 2005));
    b1.cars.push(new carModel("A6", 2005));

    var b2 = new brandModel("Volkswagen");
    b2.cars.push(new carModel("Golf", 2010));
    b2.cars.push(new carModel("Passat", 2008));
    b2.cars.push(new carModel("Polo", 2012));

    this.brands.push(b1);
    this.brands.push(b2);
}

function brandModel(name)
{
    this.nameOfBrand = ko.observable(name);
    this.cars = ko.observableArray();

    this.removeCar = function(car){
        this.cars.remove(car); // this.cars = undefined;
    }
}

function carModel(name, year){
    this.nameOfCar = ko.observable(name);
    this.yearOfCar = ko.observable(year);
}

ko.applyBindings(new RootModel());

Thanks